Various pulse rate detection systems are known in the art. The pulse rate detection devices comprise, for example, devices that use pressure sensitive transducers such as piezoelectric elements to detect pulse rate.
Another measuring technique is called as photoplethysmography. Photoplethysmography is an electro-optic technique of measuring the cardiovascular pulse wave found throughout the human body. The pulse wave is caused by the periodic pulsations of arterial blood volume and is measured by the changing optical absorption of radiant energy which this induces.
The measurement system usually consists of a source of radiant energy (for example, an infra-red light source), at least one detector for detecting the intensity of the radiant energy after propagation through the human body tissue and a data processing means for extracting bodily parameters such as pulse rate or oxygen concentration in the blood. For example, the use of infra-red light has certain advantages. It is relatively little absorbed in blood and in body tissue and blood volume changes are therefore observed with a reasonable contrast with relatively low radiant energy. The use of other light wavelengths is also possible. The use of photoplethysmography as a measuring technique is entirely non-invasive and can be applied to any blood bearing tissue, for example, a finger, nail, ear lobe, nose and wrist.
The light intensity varies together with changes in the volume in the tissue of interest both due to changes in the path-length between the light-source and the photo detector (due to volume changes) and due to the changes in the optical density of the tissue including blood and liquids (e.g. due to arterial blood pulsation). In a homogeneous layer of blood, the Beer-Lambert law suggests light intensity to decay exponentially as a function of distance due to light absorption. However, no tissue is homogeneous, and hence in addition to light absorption factors such as light scatter, refraction and reflection, which all depend on the exact anatomy and geometry of the tissue, also affect the measured signal. Therefore, the total amount of radiant energy reaching detector is sensitive to sensor positioning and any deformations of the tissue.
It is characteristic to a human tissue that light is highly scattered in the tissue. Therefore, a detector positioned on the surface of the skin is able to measure reflections. These reflections are variously absorbed depending on whether the light encounters weakly or highly absorbing tissue. Any change in blood volume will be registered by the detector at the surface since increasing (or decreasing) volume will cause more (or less) absorption. When the illuminated blood flow pulsates, it alters mainly the total absorption co-efficient of the tissue volume but also the optical path length and therefore modulates the light absorption throughout the cardiac cycle. Non-pulsating fluids and tissues do not modulate the light but have a constant level of absorption (assuming there is no movement or other deformation of the tissue volume).
The result of the absorption is that any light reflected from the pulsating vascular bed contains an AC component which is proportional to and synchronous with the subject's heart pumping action. It is this modulated component which is known as the photoplethysmographic signal. A plethysmographic measurement can be achieved by measurement of the intensity of radiant energy transmitted through (transmission mode systems) or reflected by (reflection mode systems) body tissue.
In theory, measuring a pulse rate is a simple measurement—at least one detector detects the intensity of the radiant energy received via human tissue and a data processing means is used to determine needed parameters from the signals received from the at least one detector. In practice, the situation is not so simple especially when the subject is moving. Movements cause deformation of the tissue volume between radiant energy source and detector which will cause changes which are not related to a heart pumping action in the detected energy in the detector. These movement artifacts may be typically manifold as compared to changes in the absorption caused by blood flow changes, and therefore complex methods have been proposed to compensate for these artifacts by processing means. One possible compensation solution is presented in EP 1 297 784 A1.
In the light of the present art, it is beneficial to minimize the amount of movement artifacts in the signal and to keep the proportion of the signal related to blood flow maximal. This may be achieved by arranging the measurement so that the tissue volume through which the radiant energy propagates before reaching the detector contains maximal relative proportion of active blood flowing vasculature, and only minimal amount of other tissue volume i.e. tissues in which blood pulsation is less significant. However, this has some complications.
First, thickness of skin and other layers of human tissue e.g. epidermis, papillary dermis and reticular dermis, varies between individuals and also between different spatial locations in the skin, and therefore the active blood containing tissue thickness and depth from skin surface is different in different individuals and/or spatial locations on skin. Furthermore, blood flow in different layers of the skin varies dynamically between different conditions so that when skin is cold blood circulation is minimal close to skin surface while with warm skin blood flow is active close to skin surface. These aspects make it challenging to provide a reliable measurement.
In addition, portable pulse rate measuring device should optimally have a small size and long battery life to be most useful for their users.
Based on the above, there is a need for solution that would take into account at least some of the above aspects and variances that affect to pulse rate measurements and that would provide a reliable pulse rate measurement.